The present invention relates to curable epoxy resin compositions having improved shear strength, peel strength, flexibility and impact properties. The epoxy resin compositions incorporate an epoxy compound having a terminal oxirane group, and the improved strength and flexibility is achieved by covalently linking an elastomer grafted polyol to the epoxy compound other than by the terminal oxirane group. In particular, the present invention relates to curable epoxy resin compositions containing hydroxyl groups along the backbone to which elastomer grafted polyols have been covalently linked by reacting the two compounds with a diisocyanate to form a diurethane linkage between the hydroxyl group of the epoxy compound and a polyol hydroxyl group of the elastomer grafted polyol.
Epoxy resins are curable adhesive compositions based on organic compounds having one or more terminal or pendent oxirane groups. The resins are typically cured by reacting the oxirane group with a primary amine. Epoxy resin adhesives and coatings are useful because of the relative ease with which they can be converted to cured thermoset products that are strong and resistant to chemical environments. Conventional epoxy resin adhesive and coatings, however, have high glass transition temperatures and are brittle and tend to be susceptible to mechanical and thermal shock.
To overcome this deficiency, elastomers have been incorporated into curable epoxy resin compositions. Typically, an elastomeric phase is incorporated into the epoxy resin, either by direct blending or by the formation in situ of agglomerate particles upon addition of the curing agent. The elastomeric phase usually requires stabilizing to maintain a uniform blend with the epoxy resin.
For example, Marubashi, U.S. Pat. No. 4,170,613 discloses an epoxy resin blended with a copolymer of chloroprene with a glycidyl ester. The chloroprene portion of the copolymer represents an elastomer to be dispersed in the epoxy resin, while the glycidyl ester is a "stabilizer" covalently bonded to the elastomer and soluble in the epoxy resin.
Adam, U.S. Pat. No. 4,524,181, also discloses an epoxy resin blended with an elastomer that is covalently linked to a stabilizer that is soluble in the epoxy resin. This patent mentions U.S. Pat. No. 3,496,250 as teaching the blending of an acrylonitrile butadiene-styrene graft polymer into an epoxy resin to provide flexibility, shear strength and impact characteristics to the cured resin.
Cuscurida, U.S. Pat. No. 4,539,378, and Hayes, U.S. Pat. No. 5,059,641, disclose acrylonitrile/styrene graft polyols copolymerized with epoxy compounds at the terminal oxirane group of the epoxy compound. This does not provide a curable epoxy resin because all of the oxirane groups are reacted with the graft polyol. Similarly, Huybrechts, U.S. Pat. No. 4,564,648, discloses an acrylic ester graft epoxy formed by copolymerization of an epoxy resin with acrylate monomers. Again, this does not result in the formation of a curable epoxy resin because all of the terminal oxirane groups are reacted with the acrylate monomers.
Morita, U.S. Pat. No. 5,082,891, discloses a curable epoxy resin having cured silicone rubber particles uniformly dispersed therein. The compound can also include a second, aliphatically unsaturated, epoxy compound covalently linked to an aliphatically unsaturated aromatic compound by an organohydrogen-polysiloxane crosslinking agent. This covalently linked ingredient is disclosed as compatibilizing the silicone rubber particles with the curable epoxy resin.
One drawback to the elastomeric phases incorporated into prior art epoxy resin compositions is that the stability and uniformity of the phase is dependent upon the curing agent and temperature. This is particularly a problem with the in situ formation of elastomeric agglomerates.
There remains a need for curable epoxy resin compositions capable of forming uniform, stable elastomeric phases when cured that provide improved peel strength, shear strength, flexibility and impact characteristics when cured.